This project is the investigation of the cellular and molecular interactions during synaptogenesis. It has focused on a factor found in the medium conditioned by cholinergic neuronal cells which aggregates the acetylcholine receptors on muscle cells. The factor was also found in extracts of embryonic brain but was not detected in adult brain, and hence is developmentally regulated. The factor has been partially purified and characterized. Medium conditioned by cholinergic neuronal cells was also discovered to stabilize acetylcholine receptors by apparently promoting their attachment to the myotube cytoskeleton. Conditioned medium also decreased the lateral mobility of myotube acetylcholine receptors and modified the distribution of the single channel kinetics of the acetylcholine receptor.